Modeling solute in clays at the nano-metric scale Part I. Application to the alkaline series

The argillite formation has been proposed as a potential host rock for deep radioactive waste disposal. These materials have very low permeability and diffusion is likely to be the predominant transfer mechanism for solute migration.The negatively charged surface of the clays induces unexpected behaviour of the ions: while the anions diffuse slower than HTO, the transport of the cations is accelerated. To explain these two antagonistic phenomena, 'effective diffusion porosity' for anions and 'surface diffusion' for cations has usually been introduced. The aim of the present study is to describe the main mechanism that takes place in the vicinity of the charged surface (nano-metric scale) and enables to quantify the behaviour of the alkaline series (from Li to Cs), i.e increase the retention and effective diffusion coefficient from lithium to caesium. This model with few inputs can quantitatively reproduced specific adsorption of monovalent cations and adsorption isotherm for Rb, Cs. Hydration and electrostatic forces seem to be the main interactions controlling the adsorption on surface for the alkaline series. (authors)